1. Field of the Invention
This invention relates generally to the detection of trace gases.
2. Description of Related Art
Trace gas detection has applications ranging from explosive and chemical weapons detection to semiconductor manufacturing and medical diagnostics, which utilize a large range of platform technologies. Optical absorption spectroscopy is one technique to measure the presence and concentration of trace gases. For many applications extreme sensitivity is required due to the low concentrations and small optical interaction (optical cross sections) of common analytes.
Absorption measurements represent one leading technique for trace gas detection. Small changes in the transmitted intensity of a probe laser beam are used to determine the presence of absorbing species in a sample. Here, “laser” refers to any coherent source of electro-magnetic radiation, including but not limited to lasers, frequency-converted laser beams, harmonic generation from laser beams, optical parametric oscillators, and difference frequency generators. For 1 mW of incident laser radiation and 1 second measurement times, absorption sensitivities approaching 10−9/cm are in principle attainable. Precise stabilization of the laser intensity is generally seen as required for this technique. In practice, a much worse sensitivity is usually achieved. The sensitivity of absorption detection of trace gases can be enhanced by placing the absorbing sample in a high-finesse optical cavity which provides multi-pass interaction between the probe beam and the sample. The light can interact with a sample each time it reflects off the cavity mirrors, which can be greater than 105 times (corresponding to cavity finesse, F˜105), as can be achieved with state of the art mirror technology in certain wavelength ranges.
A high finesse cavity is also a narrow frequency discriminator which only allows the transmission of a narrow range of frequencies. Therefore any frequency fluctuations in the probe laser are mapped onto amplitude fluctuations in the light transmitted through the cavity, which can further exacerbate the technical problem of intensity stabilization. One approach to circumventing this problem is cavity ring-down spectroscopy (CRDS), in which the time-decay curve of the intensity transmitted through the cavity is measured when the input light is terminated or the frequency of the input light is shifted away from the cavity resonance. The light intensity can be fit to a decay function, typically exponential, whose time constant (the cavity ring down time) is related to the cavity loss which includes absorption through the cavity.
One of the additional challenges in this arena is detecting target vapors that may be obscured by backgrounds with significantly stronger absorption at one or many optical wavelengths. A technique has been developed to use mass diffusion of molecules in the sample to detect weak absorbers in the presence of potentially far stronger backgrounds. This technique is however limited by the fact that diffusion coefficients of molecules vary only by the square root of the mass and the performance of the Bayesian Estimator (as described in Stockton, J. K., and A. K. Tuchman (2009)) is limited by differences in diffusion coefficients. An improved diffusion device and technique is desirable.